This invention relates generally to a hydrostatic transmission control system and, more specifically, to a hydrostatic transmission control system which is operable to effect a variation in the displacement of the hydrostatic transmission.
It is common practice to utilize mechanical and hydraulic controls to vary the effective displacement of the pump unit of a hydrostatic transmission. In addition, some electrohydraulic controls have been utilized to regulate the displacement of the hydrostatic transmission. Electrohydraulic controls have proven to be more accurate than mechanical or hydraulic controls; however, electrohydraulic controls have a higher cost associated with them. A major contributing factor to the cost of electrohydraulic controls is the number and type of hydraulic valves which are utilized in the conventional electrohydraulic control system. For example, one of the valves within the conventional electrohydraulic controls is a direct control valve or spool valve, which valves are expensive and sensitive to contaminants. In order to enhance the comparative position of electrohydraulic controls to the other known hydrostatic transmission control systems, it is desirable to minimize the number and complexity of valves utilized in the control systems without sacrificing system accuracy.
The present invention discloses a hydraulic control system which employs a reduced number of valves which are less sensitive to contaminants and simpler in design.